fix(WRK-21,WRK-28,WRK-23,IPC-30): byte-budget the DrainEvents reply, stop size errors from killing sessions

WRK-21 — DrainEvents was bounded by event count only, so a byte-heavy queue
(large string/array MxValues) built a reply above the negotiated frame maximum:
the writer rejected the frame, the exception unwound the session, and the events
already dequeued were destroyed. The drain is now byte-budgeted inside the queue
lock, so an event is dequeued only once it is known to fit and one that does not
stays at the head. Truncation is reported through the reply's existing
DiagnosticMessage (no contract change); callers drain until an empty reply. Both
reply-write seams — the control-command path and ProcessCommandAsync — now catch
MessageTooLarge and answer the correlation with an InvalidRequest reply instead
of unwinding or faulting the session. Satisfies IPC-23 R1-R3.

WRK-28 — the 10,000 drain ceiling moves to GatewayContractInfo
.MaxDrainEventsPerCommand, referenced by both the gateway request validator and
the worker clamp, replacing a comment-only sync contract. C# const only; no
.proto change.

WRK-23 — WorkerFrameWriter now peek-stamps, validates, then commits the sequence
counter immediately before the stream write, so a per-frame rejection leaves no
phantom gap on the wire.

IPC-30 — an oversized event frame stays session-fatal (it is undeliverable end to
end and neither dropping nor synthesizing a replacement is allowed), but the
death is structured: the event's identity and sizes are logged (never its value),
a WorkerFault with category PROTOCOL_VIOLATION and command method EventDrain is
written, then the session exits as before.

Docs updated in the same change: MxAccessWorkerInstanceDesign.md (drain byte cap,
truncation contract, oversized-head behavior, oversized-event policy, no control
reply is session-fatal on size), WorkerFrameProtocol.md (reply pre-sizing,
non-fatal reply-size rule, oversized-event policy, rejected frames do not consume
sequence numbers), gateway.md (DrainEvents two-axis bound).

archreview/2026-07-12/remediation/20-worker.md

@@ -16,14 +16,14 @@ members, no positional records). The worker builds and tests only on the Windows
 
 | ID | Sev | Tier | Eff | Dep | Status | Title |
 |----|-----|------|-----|-----|--------|-------|
-| WRK-21 | Medium | P0 | M | IPC-23 (same defect, fix owned here); WRK-28 (same lines) | Not started | DrainEvents bound is count-based only; an oversized reply still kills the session and loses the drained events |
+| WRK-21 | Medium | P0 | M | IPC-23 (same defect, fix owned here); WRK-28 (same lines) | Done | DrainEvents bound is count-based only; an oversized reply still kills the session and loses the drained events |
 | WRK-22 | Low | — | S | IPC-26 (same defect, fix owned here) | Not started | Cancelled `WriteAsync` leaves its frame queued; it is still written later |
-| WRK-23 | Low | — | S | WRK-21 (rejection path becomes backstop-only) | Not started | Rejected frames consume sequence numbers, producing wire gaps |
+| WRK-23 | Low | — | S | WRK-21 (rejection path becomes backstop-only) | Done | Rejected frames consume sequence numbers, producing wire gaps |
 | WRK-24 | Low | — | S | — | Not started | `AdoptNegotiatedMaxMessageBytes` has no lower-bound sanity check |
 | WRK-25 | Low | P2 | S | WRK-22 (both touch enqueue/dequeue) | Not started | WRK-12 flush coalescing never engages on the event hot path |
 | WRK-26 | Low | P1 | S | WRK-23 (soft — sequence prose); discharges IPC-29 | Not started | Write-priority and overflow doc drift from the WRK-07 change |
 | WRK-27 | Low | — | S | — | Not started | Alarm poll bypasses the watchdog's in-flight suppression (15 s vs 75 s) |
-| WRK-28 | Low | — | S | WRK-21 (land in the same commit cluster) | Not started | 10,000 drain cap is a duplicated magic constant with a comment-only sync contract |
+| WRK-28 | Low | — | S | WRK-21 (land in the same commit cluster) | Done | 10,000 drain cap is a duplicated magic constant with a comment-only sync contract |
 
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